The present invention generally pertains to slow scan television, and is particularly directed to communications of voice signals and slow scan television signals over a common communication channel.
Slow scan television systems process a video signal to enable video information to be provided and received over a narrow bandwidth voice grade communication channel, such as a telephone circuit. In order for a narrow band communication channel to be able to accomodate video information, the scanning rate of such information must be greatly reduced. Slow scan television signals typically have a scan rate of between approximately 8 and 32 seconds per video frame in order to be able to communicate sufficient video information to be able to provide a video presentation of acceptable quality on a television monitor.
Slow scan television systems are described in a U.S. Patent filed Jan. 7, 1977, by Munsey, entitled "Slow Scan Television Scan Converter", Ser. No. 758,019; now U.S. Pat. No. 4,057,836 and in a publication by Steber "SSTV to Fast Scan Converter" QST, March 1975, pp 33-40 and May 1975, pp 28-46.
In a state of the art slow scan television system, voice signals cannot be communicated over the same communication as the slow scan television signals while slow scan television signals for a given video field defining a complete picture are being communicated from one station to another. As a consequence thereof, either voice communication between the stations is not undertaken during the relatively lengthy intervals while slow scan television signals are being communicated over the communication channel, or a second communication channel is provided between the stations so as to enable voice communication while the slow scan television picture is being communicated.